Transparent organosiloxane coatings are applied to various transparent glass and plastic materials, such as optical lenses, goggles, face shields, face plates for helmets, automotive parts, etc., because of favorable properties of the coatings. One such favorable property is abrasion resistance. For transparent materials that are often soft and tend to scratch or marr quite easily, a “hard” abrasion-resistant organosiloxane coating can be applied as a protective coat over the material to prevent scratching or marring.
Organosiloxane coating compositions exhibit better stability than various other types of abrasion-resistant coating compositions, such as reactive two-component polyurethane coating systems. Reactive two-component systems need to be applied to a substrate, such as a transparent glass or plastic material, soon after mixing because the mixing initiates an irreversible reaction of the components of the coating composition, which can render the composition unsuitable for use if the composition is not used quickly enough after the initiation of the reaction. In contrast, an organosiloxane coating composition can be stored for an extended period of time after it has been prepared, i.e., weeks or months, before it is applied to the substrate.
In addition, unlike softer, more elastomeric, and less abrasion-resistant coatings, such as certain polyurethane coatings, which can have a tacky feel to the touch, the more abrasion-resistant hard organosiloxane coatings have a smooth or low-tack feel. The smooth or low-tack feel of the cured organosiloxane hard coatings enhance the cleaning properties of the coating by allowing easier wipability than the tackier soft coat coatings.
Although not an inherent property of organosiloxane coatings or a property that is easily incorporated into abrasion-resistant organosiloxane coatings, anti-fog (also referred to as anti-frost) is an additional property that is desirable for abrasion-resistant organosiloxane coatings. Fog appears when moisture condenses on a surface of the coating and is drawn into tiny droplets that scatter light. The scattering of the light gives the surface the appearance of a fog. One way a coating works to prevent fogging is to absorb the condensed moisture into the surface preventing the tiny light scattering droplets from forming. However, at some point this type of coating surface may reach saturation, thus resulting in the formation of light scattering water droplets on the surface and also resulting in poor anti-fog properties. Another way to prevent fogging is to modify the coating in a manner so as to cause the water droplets to spread or sheet across the surface, which is an effect called wetting, thereby minimizing the light scattering effect of the water droplets.
This water spreading or sheeting property can be incorporated into the coating using surface active agents, which are also known as surfactants, to improve water wetting on the surface of the coating. Surfactants generally contain both hydrophilic and hydrophobic segments and are used to impart the properties of these segments on the surfaces of the coating, including surfaces of organosiloxane polymer coatings. These surfactants can be applied externally by wiping or spraying the surfactant on the surface to form a temporary anti-fog film. Alternatively, the surfactants can be mixed in with the coating composition as an additive before it is applied to a surface, so that the surfactant becomes mixed within the physical structure of the coating, i.e., polymer structure, as the coating composition cures. Such surfactants that are only externally applied or mixed in with the coating easily wash off or leach away from the cured coating when exposed to water washing or water soaking, thereby resulting in the dissipation of the anti-fog properties for the coating surface.
Described herein are stable organosiloxane coating compositions which provide transparent, abrasion-resistant, chemical-resistant, low-tack, and water-washable anti-fog coatings when cured. The coating compositions described herein include surfactants that are resilient to water washing or water soaking.